1. Field of the Invention
The present invention relates to a tunable voltage-controlled pseudo-resistor, and in particular to a tunable voltage-controlled pseudo-resistor, that is composed mainly of series-connected PMOS elements operated in sub-threshold regions in cooperation with an auto-tuning circuit, so that the pseudo-resistor is able to overcome the variations of manufacturing process and drift of common-mode voltage, in achieving symmetric voltage-resistance characteristics, while maintaining constant resistance under high input voltage.
2. The Prior Arts
In general, the active type resistor used for testing in CMOS manufacturing process is operated in a triode region, with its aim of providing a resistance as small as possible, so that the switching circuit can be designed to lower its resistance and noise. Usually, in an analog front-end circuit for biomedical or audio application, a very large resistance is required to achieve an ultra-low frequency pole, to filter out the noise outside the operation frequency. However, regardless of using off-the-shelf devices, or standard cells on chips, this approach of providing high resistance is not very cost effective. The design of placing passive elements on chip is a feasible solution, but it occupies quite a large area. In addition, to connect MOS transistors of long length in series is able to achieve high resistance, yet that still has to occupy quite a large area and instability in process variations, if the ultra-low frequency is required.
In reference document 1, a pseudo-resistor is realized through connecting diodes. Though this approach can realize high resistance required, yet it has the unsymmetric voltage-resistance characteristics (for example, when input voltage level >0, and the signal swing is low, the equivalent resistance is quite large; and when voltage level <0, it means that the reverse current flow is occurred, and the equivalent resistance is reduced drastically), and when it is subjected to DC drifting of common-mode voltage (DC voltage or voltage across two ends of a resistor), the resistance obtained is reduced significantly.
In reference document 2, it is pointed that, in case high voltage is applied on gate of a transistor, the transistor can provide fairly high resistance, but since its resistance can not be adjusted, its application is rather limited.
In reference documents 3 and 4, it is further pointed out that, if the voltage applied on a gate of a transistor, and voltages in-between the series-connected transistors are kept constant, the pseudo-resistor circuit can provide symmetric voltage-resistance characteristic (namely, when voltage >0 or voltage <0, current can flow from left to right, or current can flow from right to left, such that high resistance can be obtained). In other words, this kind of design can overcome the drift of common-mode voltage, but it still affected by the limitations that range of input voltage is not wide enough.
In particular, for reference document 3, in case the circuit controlling the gate voltage of the transistor is designed to be a source follower circuit, then the variations of voltage across two ends of the pseudo-resistor (namely, the drifting of the common-mode voltage) can not be linearly (singly) detected and followed due to the limited linear operation of the source follower circuit, so that in an analog front-end circuit of biomedical application, a stable cutoff frequency can not be obtained in large input swing. The shortcomings of reference documents 3 and 4 are that, the range of input voltage is rather small, and an external control circuit is required.
In view of the problems and shortcomings of the prior art, the design and development of a pseudo-resistor capable of providing stable and extra-high resistance while increasing input voltage range, is an urgent task in this field.